What is a menstrual cycle?

The menstrual cycle is
the series of changes a woman's body goes through to prepare for a pregnancy.
About once a month, the
uterus grows a new lining (endometrium) to get ready
for a fertilized egg. When there is no fertilized egg to
start a pregnancy, the uterus sheds its lining. This is the monthly
menstrual bleeding (also called menstrual period) that women have from their early
teen years until
menopause, around age 50.

The menstrual
cycle is from Day 1 of bleeding to Day 1 of the next time of bleeding. Although
the average cycle is 28 days, it is normal to have a cycle that is shorter or longer.

Girls usually start having menstrual periods between the ages of 11 and
14. Women usually start to have fewer periods between ages 39 and 51. Women in
their 40s and teens may have cycles that are longer or change a lot. If you are
a teen, your cycles should even out with time. If you are nearing menopause,
your cycles will probably get longer and then will stop.

Talk to
your doctor if you notice any big change in your cycle. It's especially
important to check with your doctor if you have three or more menstrual periods that last
longer than 7 days or are very heavy. Also call if you have bleeding between
your periods or pelvic pain that is not from your period.

What controls the menstrual cycle?

Your hormones
control your menstrual cycle. During each cycle, your brain's
hypothalamus and
pituitary gland send hormone signals back and forth
with your
ovaries. These signals get the ovaries and uterus
ready for a pregnancy.

The hormones
estrogen and
progesterone play the biggest roles in how the uterus
changes during each cycle.

Estrogen builds up the lining of the
uterus.

Progesterone increases after an ovary releases an egg
(ovulation) at the middle of the cycle. This helps the
estrogen keep the lining thick and ready for a fertilized egg.

A
drop in progesterone (along with estrogen) causes the lining to break down.
This is when your period starts.